Electrowinning manganese



July 15, 1969 Filed Oct. 19. 1965 DENSITY MA/SQ m DENSITY MA /in 2Sheets-Shoot 2 25o m L60 mq/L 0 I I I I I CATHODE POTENTIAL,VOLTS Iso-|ook '0 v I l 1 I o 2 4 e a so we :4 '6

mg/L INVENTOR F G 4 C.L.Montell 8 G.R.Ferment BYM ATTORNEYS UnitedStates Patent US. Cl. 204-105 6 Claims ABSTRACT OF THE DISCLOSURE In theelectrowinning of manganese zinc ions are added to a catholyte in amanganese deposition cell to reduce the current density necessary toproduce pure manganese.

The present invention relates generally to electrowinning manganese andmore particularly to a process wherein zinc is added to the catholyte ina manganese deposition cell to reduce the current density necessary toachieve manganese electrolytically.

The production of pure manganese (99.9+%) by electrolysis requires verypure manganese to be in a solution that is fed to the electrolytic cell.If the impurities that usually accompany manganese ore, such as cobalt,nickel, molybdenum and copper are present to any great extent, manganesecan not be deposited on the cathode of an electrolytic cell. Actually,the significant presence of .any of these impurities causes whatevermanganese that is plated on the cathode to recede from it, i.e. go backinto solution. Even in the total absence of these impurities the cellmust be operated at or above a minimum current density and minimumvoltage to achieve manganese deposition. With the addition of thespecified impurities even in trace amounts, the current density requiredfor deposition is substantially increased and is accompanied by a slightincrease in the voltage necessary for deposition. Hence, to provide themost efficient method of obtaining manganese, i.e. the method requiringminimum current density and minimum voltage for deposition, it hasalways previously been thought that the catholyte feed to theelectrolytic cell must be as free as possible of all metallicimpurities.

We have found, unexpectedly and through experimentation, that theaddition of trace amounts of zinc to the catholyte actually enhances thedeposition of manganese, both in pure solutions and in solutions havingtrace amounts of the impurities usually accompanying the raw ore. Theaddition of zinc, in trace amounts, reduces the minimum current densityrequired for manganese to be deposited on the cathode, both when thecatholyte solution is pure and when it contains impurities. This resultoccurs since it has been found that zinc actually aids in the depositionof manganese at the cathode of the cell. Hence, the present inventionenables input requirements of the deposition cell to be reduced, wherebyincreased operating eiiiciency is achieved. It is also expected thatcertain ores can be processed without having a filter to remove theusual impurities that accompany manganese ore, such as cobalt, nickel,molybdenum and copper. In addition, the present invention enablesdeposition to occur at a higher rate than previously if the cell isoperated at the same current density.

It is, accordingly, an object of the present invention to provide a newand improved method for electrowinning manganese, which method is moreefficient in the use of electric power than prior methods and therebyless expensive in operation.

Another object of the invention is to provide a method of electrowinningmanganese by lowering the minimum current density required fordeposition.

Another object of the invention is to provide an electrolytic cellhaving a catholyte that enables manganese to be deposited at lowercurrent densities than with prior art cells.

A further object of the invention is to provide a method of reducing thepower input requirements for the electrolytic fabrication of puremanganese by adding trace amounts of zinc to the catholyte feed to thecell.

An additional object of the invention is to provide a method ofelectrolytically manufacturing pure manganese wherein the deleteriouseffects, on the deposition process, of trace amounts of the impuritiesthat accompany the manganese ore are overcome.

Still an additional object of the invention is to provide a method ofelectrowinning manganese from certain ores without the necessity forprecipitating and filtering trace impurities from them.

Yet a further object of the invention is to provide a new and improvedmethod for electrowinning manganese at a higher efficiency thanpreviously possible with the same input power.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of one specific embodiment thereof,especially when taken in conjunction with the accompanying drawings,wherein:

FIGURE 1 is a schematic diagram of a typical manganese cell with whichthe present invention is adapted to be utilized;

FIGURES 2 and 3 are graphs of current density versus voltage fordifferent catholyte solutions applied to the cell of FIGURE 1; and

FIGURE 4 is a plot 0t current density required for manganese depositionversus zinc concentration in the catholyte.

Reference is now made to FIGURE 1 of the drawings wherein electrolyticcell 11 is divided into anolyte chamber 12 and a catholyte chamber 13 bydiaphragm 14 which may be canvas or any other suitable liquid permeablematerial. Chambers 12 and 13 communicate with each other by overflowpipe 15 that enables the catholyte liquid from chamber 12 to flow intochamber 13.

Variable D.C. potential source 16 is connected between stainless steelcathode 17, located in catholyte chamber 12, and lead alloy anode 18,located in chamber 13, to polarize the anode positively relative to thecathode.

The catholyte in chamber 12 is a solution of the usual concentration ofapproximately 35 grams per liter of ionic manganese (Mn++),approximately grams per liter of ammonium sulphate [(NH SO andapproximately 0.1 gram per liter sulphur dioxide (S0 to which is addedtrace amounts of ionic zinc (Zn++) in concentrations of between 0.15 to5.0 milligrams per liter. While the maximum zinc concentration can beseveral times the amount stated, the addition of zinc above aconcentration of 5.0 milligrams per liter is wasteful. The temperatureof the catholyte solution is approximately 30 centigrade and the pHthereof is about 7.20. The anolyte in chamber 13 is the usual solutionof approximately 15 grams per liter of manganous sulphate, 30 grams perliter of sulphuric acid, and approximately grams per liter of ammoniumsulphate. The anolyte has a pH of between 1.2 and 1.6.

Anolyte is continuously drawn from chamber 13 by pump 19 that feeds theliquid solution through filter 21 to leaching tank 22. In tank 22,manganese ore, in the form of conditioned manganese oxide (MnO), istreated with the filtered anolyte and a leaching solution of ammoniumsulphate and sulphuric acid. The solution emerging from tank 22 ispassed through filter 23 which removes metallic impurities that areusually present in the manganese ore and which have not previously beenWithdrawn from the manganese oxide applied to leaching tank 22. Typicalof the impurities that are sometimes withdrawn are: iron, arsenic,antimony, tin, lead, nickel, cobalt, molybdenum, silica, aluminum,calcium and magnesium, although cobalt, nickel, molybdenum and copperare the only impurities present usually to an extent that will causedetrimental results to occur in the manganese deposition. In any event,it is to be noted that zinc is not an impurity found in manganese ores.

Downstream of filter 23, pure zinc is added to the catholyte feedsolution in trace quantities so that the catholyte solution in chamber12 has a concentration of Zn++ ions between 0.15 milligram per liter and5.0 milligrams per liter. If the upper limit is exceeded, the manganesedeposited on cathode 17 does not achieve the 99.9% purity expected fromelectrolytic processing. Zinc is shown as being introduced into thesystem downstream of filter 23 because it is difiicult, -with certaintypes of filters, to separate it from the natural impuritiesaccompanying the manganese ore. It is to be understood that zinc may,however, be introduced into the system upstream of filter 23 if suitablefilters are available. After zinc, in suitable quantities, has beenadded to the catholyte feed solution, the solution is fed into chamber12 by pump 24.

To operate cell 11 in the most eflicient manner possible, it isnecessary to monitor the current density therein, as feed to the cell bysource 16, in addition to the polarization voltage of cathode 17relative to the negative terminal of the source. Current density ismeausred by connecting D.C. ammeter 24 in series with'source 16 and oneof the cell electrodes, preferably anode 18 so that the cathodepolarization voltage can be more accurately measured. Since the area ofelectrode 17 is constant, the reading of meter 24 derives a directindication of cathode current density. The current feed to cell 11 bysource 16 and the cathode polarization voltage must be monitored so thatthe cell can be operated at current densities and polarization voltagesabove the minimum values required to initiate and maintain manganesedeposition on cathode 17.

The polarization voltage is monitored by connecting salt bridge 25 to aportion of cathode 17 that is always submerged in the catholyte.Connected in series with salt bridge 25 is saturated calomel electrode26 that includes a fiber junction. Electrode 26 is utilized as areference electrode for measuring the voltage of cathode 17 Standardcell 27, having a voltage on the order of 1.018 volts, is connectedbetween the electrode 26 and voltmeter 28 to buck the voltage from thesaturated calomel electrode. With this arrangement, the net voltage dropacross the measuring circuit is minimized, whereby current flow throughit is virtually zero and accurate measurements are derived withouttaking significant power from source 16.

To provide an understanding of the operation and advantages of thepresent invention, reference is now made to FIGURES 2-4. In FIGURES 2and 3, current density in catholyte chamber 12 is plotted againstcathode polarization voltage, i.e. the voltage between the negativeterminal of source 16 and salt bridge 25.

Curve 31, FIGURE 2, is a plot of current density versus polarizationvoltage when the system operation is such that filter 23 has functionedperfectly, whereby no impurities are in the catholyte feed and no zinchas been introduced therein. Under these conditions, deposition ofmanganese occurs only on the portion of curve 31 at point 32 and to theright thereof, along line 33. If the cathode polarization voltage isless than 1.4 volts and the current density of the catholyte in chamber12 is less than approximately 135 milliamps per square inch, manganesecan not be deposited on cathode 17 It is to be noted that both thecurrent and voltage parameters must be reached and maintained fordeposition to occur; if only one of these parameters prevails, manganesewill not be coated onto cathode 17. Instead, manganese will recede fromcathode 17 into the catholyte solution and thereby nullify the energypreviously used for deposition.

Since the voltage required to deposit manganese is substantially fixedby the electrolyte that must be employed, the present invention seeks todecrease the input current required for manganese deposition and therebyincrease the efficient use of the electric power supplied to the systemby source 16. Decreases in the current density required for depositionhave been found to occur if trace amounts of Zn++ ions are added to thecatholyte. The decreased current densities required for deposition areillustrated by curves 34-37, FIGURE 2, wherein the concentrations of Znions in the catholyte are respectively 0.31 milligram per liter, 0.63milligram per liter, 5.00 milligrams per liter and 15 milligrams perliter. It is noted that the minimum current densities required fordeposition, i.e. the points where curves 34-37 intersect line 33, areapproximately milliamps per square inch, 100 milliamps per square inch,90 milliamps per square inch and 85 milliamps per square inch,respectively for the four different concentrations. These results arealso reflected in the curve of FIGURE 4. Hence, the minimum currentdensities required for manganese deposition are reduced almost inone-half from milliamps per square inch to approximately 85 milliampsper square inch by adding trace amounts of Zn ions to the catholytesolution and increased operating efiiciency is thereby attained.

Another advantage occurring from the addition of trace amounts of zincto the catholyte is that for a specified current density depositionoccurs at a more rapid rate than if zinc had not been added, providedthe minimum voltage and current density requirements are met. This isascertained from the straight lined portion 33 of FIGURE 2 sincedeposition rate is a function of distance along line 33 from the pointat which the line intersects curves 31 and 34-37.

The farther from the intersection point along line 33 one moves, themore rapid the deposition rate becomes. Hence, at point 32, the currentand voltage are such that the tendency of manganese to recede fromcathode 17 is just overcome and deposition occurs at a slow rate when nozinc has been added to the catholyte solution. In contrast, point 32 issignificantly removed from the intersection of curves 34-37 with line33, evidence indicating that deposition occurs more rapidly when Zn++ions are added to the solution. Hence, for the same input powernecessary to operate the cell at point 32 considerably greater amountsof manganese are deposited during the same interval with the addition ofzinc.

It is also interesting to note the beneficial results that the additionof zinc has in compensating for the detrimental effects of theimpurities usually found in manganese ore. These detrimental effects canbe ascertained from polarization curves 41 and 42, FIGURE 3. Curve 41 isa plot of catholyte current density versus cathode polarizationpotential when the catholyte contains no impurities. In contrast, curve42 indicates how much more current is required to achieve manganesedeposition when 1.60 milligrams per liter of nickel ions (Ni++) areadded to the catholyte. Also, curves 41 and 42 provide an indication, bythe significant area between them, of the considerably greater powerrequired to bring cell 11 from its rest condition of zero polarizationvoltage and zero current, when a trace amount of nickel is added.

The adverse effects of the nickel impurity can be compensated and evenovercome by the addition of sufiicient quantities of zinc. This isindicated by curves 43-45, wherein Ni++ ion concentration is maintainedsubstantially constant at 2.00 milligrams per liter, 2.50 milligrams perliter and 5.00 milligrams per liter, respectively.

Similar advantageous results were found to exist when trace amounts ofZn++ ions were introduced into cat-holytes containing trace amounts ofcobalt ions (Co++), copper ions (Cu++), and cadmium ions (Cd++), asindicated by the following table.

TABLE Minimum Deposi Additive to pure Concentration, tion current,electrolyte n1g./L. IXlZL/Sq. in.

The tabulated results, indicated by the table and FIG- URE 3, show thatwith the addition of trace amounts of zinc, according to the presentinvention, the use of certain ores having low impurity content enablesfilter 23 to be eliminated completely.

While we have described and illustrated one specific embodiment of ourinvention, it will be clear that variations of the details ofconstruction which are specifically illustrated and described may bemade without departing from the true spirit and scope of the inventionas defined in the appended claims.

We claim:

1. In the process of electrowinning manganese in an electrolytic cellhaving a cathode in a catholyte and an anode in an anolyte, saidcatholyte and anolyte including manganese sulfate and ammonium sulfate,comprising the steps of applying sufficient DC. power to said cathodeand anode to maintain the anode-cathode polarization voltage and cathodecurrent density at least at predetermined levels required to initiateand maintain the deposition of manganese on the cathode, adding traceamounts of zinc ions to a solution including manganese ions, and feedingthe solution including the zinc and manganese ions to the catholytewhile said voltage and current density are maintained at least at saidpredetermined levels said trace amounts of zinc ions being of sufficientquantity to reduce the minimum current density at which manganesedeposition occurs on the cathode while the minimum polarization voltagefor deposition remains constant.

2. The process of claim 1 wherein said trace amounts of zinc aresufficient to provide an ionic zinc concentration in excess of 0.15milligram per liter in the catholyte.

3. The process of claim 1 wherein the amount of zinc added is such thatthe concentration of ionic zinc in the catholyte is between 0.15milligram per liter and 5.0 milligrams per liter.

4. The process of claim 1 wherein the solution includes trace amounts ofcobalt and/or nickel which normally raise the minimum current densityrequired for manganese deposition and the quantity of zinc ions in thesolution is sutficient to compensate for the rise in minimum currentdensity required for deposition.

5. The process of claim 4 wherein said trace amounts of zinc aresufi'icient to provide an ionic zinc concentration in excess of 0.15milligram per liter in the catholyte.

6. The process of claim 4 wherein the amount of zinc added is such thatthe concentration of ionic zinc in the catholyte is between 0.15milligram per liter and 5.0 milligrams per liter.

References Cited sition of Manganese, Bureau of Mines, Report ofInvestigations, May 1946, US. Dept. of the Interior.

JOHN H. MACK, Primary Examiner H. M. FLOURNOY, Assistant Examiner

